Antennas have been long used as devices for converting a high-frequency current into an electromagnetic ray and an electromagnetic ray into a high-frequency current. The antennas are categorized into subgroups such as linear antennas, planar antennas, and solid antennas, based on their shapes. The linear antennas are further categorized into subgroups such as a dipole antenna, a monopole antenna, and a loop antenna, based on their structures.
The dipole antenna is a linear antenna having a very simple structure and is widely used as a base-station antenna to this day. The monopole antenna, which requires only half a length of the dipole antenna, is frequently used as an antenna for a mobile device.
In principle, the monopole antenna and the loop antenna require bottom boards infinitely extended. However, in a mobile device with a limited space, it is difficult to provide a bottom board having a sufficient size. Also, in a case where a metal member is provided near the antenna, an input impedance of the antenna is greatly changed. This gives rise to a problem that the antenna and a feed line cannot be matched in impedance.
The patent literature 1 discloses an art that stabilizes an input impedance by use of an electric conductor pattern provided on a planar sheet and a bottom board facing the electric conductor pattern. The patent literature 2 discloses an antenna in which a reflective plate of a display or a display frame serves as a bottom board so that it is not necessary to independently provide a bottom plate.